The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for forming a buried word line in a semiconductor device.
In a fabricating process for a small DRAM such as a 60 nm DRAM, it is essential to form a buried word line in order to increase the integration degree of a transistor in a cell, simplify a fabricating process, and improve a device property such as a current leakage property.
In general, a trench is formed and a word line is buried in the trench to form a buried word line. Such a buried word line forming method according to the related art has several advantages. For example, interference between a bit line and a word line is minimized, the number of stacked films is reduced, and a refresh property is improved by reducing the overall capacitance of cells.
However, the buried word line forming method according to the related art disadvantageously increases a sheet resistance (Rs) of the buried word line if a single layer of a titanium nitride (TiN) layer is used. In order to reduce such an increase of the sheet resistance, a bi-layer of a titanium nitride (TiN) layer and a tungsten (W) layer was used.
However, although the sheet resistance can be reduced by using the bi-layer of a titanium nitride (TiN) layer and a tungsten (W) layer, it causes a gap-fill problem because a width of a buried word line shrinks.
Further, the refresh property can be improved by increasing a depth of an etch-back process for forming a buried word line. However, it also increases the sheet resistance of the buried word line.
FIG. 1 is a graph showing a sheet resistance increase phenomenon that occurs when an etch-back depth is increased or when a bi-layer is used for a buried word line.
Referring to FIG. 1, a first bar 20A shows a sheet resistance when a buried word line is formed using a single layer of a titanium nitride layer and an etch-back depth of 500 Å. As shown, the sheet resistance of the first bar 20A is comparatively large. A second bar 20B shows a sheet resistance when a buried word line is formed using a bi-layer of a titanium nitride layer and a tungsten layer and an etch-back depth of 500 Å. As shown, the sheet resistance of the second bar 20B is reduced compared to the sheet resistance of the first bar 20A. A third bar 20C shows a sheet resistance when a buried word line is formed using a bi-layer of a titanium nitride layer and a tungsten layer and an etch-back depth of 650 Å. As shown, the sheet resistance of the third bar 20C is increased compared to the sheet resistance of the second bar 20B.